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FS-8700-24 ControlNet Driver Manual Page 11 of 12
Appendix A. Advanced Topics
Appendix A.1. Write-throughs
Some FieldServer drivers support “write-throughs”. This is the ability of a Read Map
Descriptor to automatically create a temporary write Map Descriptor to update the value of
an external R/W (Read or Write) memory location. Since the ControlNet card contains
separate Read-Only (Output) and Write-Only (Input) Buffers, write-throughs are not
supported on ControlNet messaging, but are supported for PCCC messaging. This is
because PCCC messages are embedded within ControlNet messages allowing for the
receiving device to process the embedded write message.
Appendix A.2. Map Descriptor functions
The diagram below explains the difference between the use of WRBC (Write Block
Continuous) and RDBC (Read Block Continuous) Map Descriptors. Note that WRBC Map
Descriptors write data from the FieldServer’s Data Arrays into the output data memory area
of the card whereas RDBC Map Descriptors place data from the card’s input data memory
area into the FieldServer’s Data Arrays. The IO data memory areas are kept in RAM on the
card and are defined by Connection Control Blocks on the card.
Data Array
WRBC Mapdesc
Data Array
RDBC Mapdesc
Input data memory
area
Output data memory
area
Output data memory
area
Input data memory
area
ControlNet
Network
Note:
1. For ControlNet messaging the target connection is specified
by Node_Name & CN_Instance.
2. For PCCC messaging the target connection is specified by
Node_Name & Block_Number.
SST 5136-CN Card
.
.
.
Input data memory
area
Output data memory
area
ControlNet Device
Connection
Control
Block 01
Connection
Control
Block 128
FieldServer
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